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  <xs:element id="next_SeniorSecuredNotesMember" abstract="true" name="SeniorSecuredNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_SeniorSecuredLoansMember" abstract="true" name="SeniorSecuredLoansMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_CDCreditAgreementMember" abstract="true" name="CDCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_TCFCreditAgreementMember" abstract="true" name="TCFCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_Train4LLCCreditAgreementMember" abstract="true" name="Train4LLCCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A6.67SeniorSecuredNotesMember" abstract="true" name="A6.67SeniorSecuredNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A6.85SeniorSecuredNotesMember" abstract="true" name="A6.85SeniorSecuredNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A6.58SeniorSecuredNotesMember" abstract="true" name="A6.58SeniorSecuredNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A6.72SeniorSecuredLoansMember" abstract="true" name="A6.72SeniorSecuredLoansMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A7.11SeniorSecuredLoansMember" abstract="true" name="A7.11SeniorSecuredLoansMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A6.56SeniorSecuredNotesMember" abstract="true" name="A6.56SeniorSecuredNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A13.00SuperFinCoTermLoanMember" abstract="true" name="A13.00SuperFinCoTermLoanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A8.00ARCorporateCreditAgreementMember" abstract="true" name="A8.00ARCorporateCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A13.50ARCorporateCreditAgreementMember" abstract="true" name="A13.50ARCorporateCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_Phase1LLCMember" abstract="true" name="Phase1LLCMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_Train4LLCMember" abstract="true" name="Train4LLCMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_Train5LLCMember" abstract="true" name="Train5LLCMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_Phase1LLCCreditAgreementMember" abstract="true" name="Phase1LLCCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_LineOfCreditPeriodAfterWhichAmortizationBegins" abstract="false" name="LineOfCreditPeriodAfterWhichAmortizationBegins" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="next_Train5LLCCreditAgreementMember" abstract="true" name="Train5LLCCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_FinCoCreditAgreementMember" abstract="true" name="FinCoCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_VariableRateComponentAxis" abstract="true" name="VariableRateComponentAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="next_VariableRateComponentDomain" abstract="true" name="VariableRateComponentDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_VariableRateComponentOneMember" abstract="true" name="VariableRateComponentOneMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_VariableRateComponentTwoMember" abstract="true" name="VariableRateComponentTwoMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_VariableRateComponentThreeMember" abstract="true" name="VariableRateComponentThreeMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_SuperFinCoTermLoanMember" abstract="true" name="SuperFinCoTermLoanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_Train5CreditAgreementMember" abstract="true" name="Train5CreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_SuperHoldingsMember" abstract="true" name="SuperHoldingsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_LineOfCreditOptionToExtendPeriod" abstract="false" name="LineOfCreditOptionToExtendPeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="next_LineOfCreditExtensionOptionAvailablePeriod" abstract="false" name="LineOfCreditExtensionOptionAvailablePeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="next_DebtInstrumentInterestInKindPercentage" abstract="false" name="DebtInstrumentInterestInKindPercentage" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="next_NumberOfTranches" abstract="false" name="NumberOfTranches" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="next_DebtInstrumentPercentageOfInterestPayableInCashOrInKind" abstract="false" name="DebtInstrumentPercentageOfInterestPayableInCashOrInKind" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="next_DebtInstrumentPercentageOfInterestPayableInCash" abstract="false" name="DebtInstrumentPercentageOfInterestPayableInCash" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="next_DebtInstrumentCovenantHistoricalDebtServiceCoverageRatioMinimum" abstract="false" name="DebtInstrumentCovenantHistoricalDebtServiceCoverageRatioMinimum" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <xs:element id="next_PhaseOneMember" abstract="true" name="PhaseOneMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_TrainFiveMember" abstract="true" name="TrainFiveMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_FinCoSwapsMember" abstract="true" name="FinCoSwapsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_WarrantsExercisePriceOneMember" abstract="true" name="WarrantsExercisePriceOneMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_WarrantsExercisePriceTwoMember" abstract="true" name="WarrantsExercisePriceTwoMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_WarrantsDue2031Member" abstract="true" name="WarrantsDue2031Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_WarrantsDue2032Member" abstract="true" name="WarrantsDue2032Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_ExercisePeriodAxis" abstract="true" name="ExercisePeriodAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="next_ExercisePeriodDomain" abstract="true" name="ExercisePeriodDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_PeriodOneMember" abstract="true" name="PeriodOneMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_PeriodTwoMember" abstract="true" name="PeriodTwoMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_ClassOfWarrantOrRightNumberOfWarrantsPotentiallySubjectToCashExercise" abstract="false" name="ClassOfWarrantOrRightNumberOfWarrantsPotentiallySubjectToCashExercise" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="next_ClassOfWarrantOrRightVolumeWeightedAveragePricePeriod" abstract="false" name="ClassOfWarrantOrRightVolumeWeightedAveragePricePeriod" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="next_VolumeWeightedAverageTradingPriceThreshold" abstract="false" name="VolumeWeightedAverageTradingPriceThreshold" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:perShareItemType"/>
  <xs:element id="next_SeriesAExchangeOptionMember" abstract="true" name="SeriesAExchangeOptionMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_AccountsPayableAcquisitionOfPropertyPlantAndEquipment" abstract="false" name="AccountsPayableAcquisitionOfPropertyPlantAndEquipment" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="next_AccruedLiabilitiesAcquisitionOfPropertyPlantAndEquipment" abstract="false" name="AccruedLiabilitiesAcquisitionOfPropertyPlantAndEquipment" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="next_NoncashOrPartNoncashSettlementOfWarrantLiabilities" abstract="false" name="NoncashOrPartNoncashSettlementOfWarrantLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="next_CapitalizedInterestPaidInKind" abstract="false" name="CapitalizedInterestPaidInKind" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
</xs:schema>
