Exhibit 10.15


SUPPLEMENTAL
GUARANTEE AGREEMENT


Supplemental Guarantee
Agreement (this “Supplemental Guarantee
Agreement”), dated as of September 21, 2006, among Reliant
Energy Power Supply, LLC (the “Guaranteeing
Subsidiary”), a Delaware limited liability company, a subsidiary of
Reliant Energy, Inc. (or its permitted successor), a Delaware corporation (the “Company”), the Company, the other Subsidiary
Guarantors (as defined in the Guarantee Agreement referred to herein) and J.P.
Morgan Trust Company, National Association, as trustee under the Indenture (as
defined in the Guarantee Agreement referred to below) (the “Trustee”).


WITNESSETH


WHEREAS, the Company and the Subsidiary Guarantors have
heretofore executed and delivered to the Trustee a Guarantee Agreement (the “Guarantee Agreement”), dated as of December 22, 2004, providing
for the Company’s guarantee (the “REI Guarantee”)
of the Pennsylvania Economic Development Financing Authority’s (“PEDFA”) Exempt Facilities Revenue Bonds (Reliant Energy
Seward, LLC Project), Series 2002A (the “Series 2002A Bonds”),
and the Subsidiary Guarantors’ guarantees of the REI Guarantee;


WHEREAS, the Guarantee Agreement provides that under
certain circumstances the Guaranteeing Subsidiary shall execute and deliver to
the Trustee a supplemental guarantee agreement pursuant to which the
Guaranteeing Subsidiary shall unconditionally guarantee all of the Company’s
Obligations under the REI Guarantee and the Guarantee Agreement (the “Subsidiary Guarantee”); and


WHEREAS, pursuant to Section 9.01 of the Guarantee
Agreement, the Trustee, the Company and the other Subsidiary Guarantors are
authorized to execute and deliver this Supplemental Guarantee Agreement.


NOW THEREFORE, in consideration of the foregoing and
for good and valuable consideration, the receipt of which is hereby
acknowledged, the Guaranteeing Subsidiary, the Trustee, the Company and the
other Subsidiary Guarantors mutually covenant and agree for the equal and
ratable benefit of the Holders of the Series 2002A Bonds as follows:


1.             Capitalized Terms.  Unless otherwise defined in this Supplemental
Guarantee Agreement, capitalized terms used herein without definition shall
have the meanings assigned to them in the Guarantee Agreement.


2.             Agreement to be Bound;
Guarantee.  The Guaranteeing
Subsidiary hereby becomes a party to the Guarantee Agreement as a Subsidiary Guarantor
and as such will have all of the rights and be subject to all of the
Obligations and agreements of a Subsidiary Guarantor under the Guarantee
Agreement.  The Guaranteeing Subsidiary
hereby agrees to be bound by all of the provisions of the Guarantee Agreement applicable
to a Subsidiary Guarantor and to perform all of the Obligations and agreements
of a Subsidiary Guarantor under the Guarantee Agreement.  In furtherance of the foregoing, the
Guaranteeing Subsidiary shall be deemed a Subsidiary Guarantor for purposes of
Article 12 of the Guarantee Agreement, including, without limitation, Section
12.02 thereof.



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3.             NEW YORK LAW TO GOVERN. 
THE INTERNAL LAW OF THE STATE OF NEW YORK SHALL GOVERN AND BE USED TO
CONSTRUE THIS SUPPLEMENTAL GUARANTEE AGREEMENT BUT WITHOUT GIVING EFFECT TO
APPLICABLE PRINCIPLES OF CONFLICTS OF LAW TO THE EXTENT THAT THE APPLICATION OF
THE LAWS OF ANOTHER JURISDICTION WOULD BE REQUIRED THEREBY.


4.             Counterparts.  The parties may sign any number of copies of
this Supplemental Guarantee Agreement. 
Each signed copy shall be an original, but all of them together
represent the same agreement.


5.             Effect of Headings.  The Section headings herein are for
convenience only and shall not affect the construction hereof.


6.             The Trustee.  The Trustee shall not be responsible in any
manner whatsoever for or in respect of the validity or sufficiency of this
Supplemental Guarantee Agreement or for or in respect of the recitals contained
herein, all of which recitals are made solely by the Guaranteeing Subsidiary
and the Company.


7.             Ratification of Guarantee
Agreement; Supplemental Guarantee Agreement Part of Guarantee Agreement.  Except as expressly amended hereby, the Guarantee
Agreement is in all respects ratified and confirmed and all the terms,
conditions and provisions thereof shall remain in full force and effect.  This Supplemental Guarantee Agreement shall
form a part of the Guarantee Agreement for all purposes, and every Holder of Series
2002A Bonds heretofore or hereafter authenticated and delivered shall by bound
hereby.



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IN WITNESS WHEREOF, the parties hereto have caused
this Supplemental Guarantee Agreement to be duly executed and attested, all as
of the date first above written.


Dated : September 21, 2006.


	
  

  	
   

  	
  RELIANT ENERGY POWER SUPPLY,
  LLC

  
	
   

  	
   

  	
   

  
	
   

  	
   

  	
   

  
	
   

  	
   

  	
  By:

  	
  /s/ Lloyd A.
  Whittington

  
	
   

  	
   

  	
  Name: 

  	
  Lloyd A.
  Whittington

  
	
   

  	
   

  	
  Title: 

  	
   Vice President and Treasurer

  
	
   

  	
   

  	
   

  
	
   

  	
   

  	
  RELIANT ENERGY,
  INC.

  
	
   

  	
   

  	
   

  
	
   

  	
   

  	
   

  
	
   

  	
   

  	
  By: 

  	
   /s/ Andrew C. Johannesen

  
	
   

  	
   

  	
  Name: 

  	
   Andrew C. Johannesen

  
	
   

  	
   

  	
  Title: 

  	
   Vice President and Asst. Treasurer

  
						



 



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  RELIANT ENERGY ASSET MANAGEMENT, LLC

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY BROADBAND, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY CALIFORNIA HOLDINGS, LLC

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY COMMUNICATIONS, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY COOLWATER, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY CORPORATE SERVICES, LLC

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY ELLWOOD, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY ETIWANDA, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY FLORIDA, LLC

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY KEY/CON FUELS, LLC

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY MANDALAY, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY NORTHEAST GENERATION, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY NORTHEAST HOLDINGS, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY ORMOND BEACH, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY POWER GENERATION, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY RETAIL HOLDINGS, LLC

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY SABINE (TEXAS), INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY SERVICES DESERT BASIN, LLC

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY SERVICES MID-STREAM, LLC

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY SEWARD, LLC

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY TRADING EXCHANGE, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY VENTURES, INC.

  
	
   

  	
   

  	
   

  	
   

  	
  RELIANT ENERGY WHOLESALE GENERATION, LLC

  
	
   

  	
   

  	
   

  	
   

  	
   

  
	
   

  	
   

  	
   

  	
   

  	
   

  
	
   

  	
   

  	
   

  	
   

  	
  By:

  	
  /s/ Andrew C. Johannesen

  	
   

  
	
   

  	
   

  	
   

  	
   

  	
  Name: 

  	
  Andrew C. Johannesen

  
	
   

  	
   

  	
   

  	
   

  	
  Title: 

  	
  Assistant Treasurer of the corporations and limited

  
	
   

  	
   

  	
   

  	
   

  	
   

  	
  liability companies, and of the general partners of
  the limited partnerships, listed above

  
								



 



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  RELIANT ENERGY SERVICES, INC.

  
	
   

  	
   

  	
   

  
	
   

  	
   

  	
   

  
	
   

  	
   

  	
  By: 

  	
  /s/ Andrew C. Johannesen

  	
   

  
	
   

  	
   

  	
  Name: 

  	
  Andrew C. Johannesen

  
	
   

  	
   

  	
  Title:  

  	
  Vice President and Treasurer

  
	
   

  	
   

  	
   

  
	
   

  	
   

  	
  RELIANT ENERGY RETAIL SERVICES, LLC

  
	
   

  	
   

  	
  RELIANT ENERGY ELECTRIC SOLUTIONS, LLC

  
	
   

  	
   

  	
  RELIANT ENERGY SOLUTIONS EAST, LLC

  
	
   

  	
   

  	
   

  
	
   

  	
   

  	
   

  
	
   

  	
   

  	
   

  
	
   

  	
   

  	
  By: 

  	
  /s/ Lloyd A. Whittington

  	
   

  
	
   

  	
   

  	
  Name: 

  	
  Lloyd A. Whittington

  
	
   

  	
   

  	
  Title:

  	
  Vice President and Treasurer of the limited 

  
	
   

  	
   

  	
  liability companies listed above

  
									



 



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  RELIANT ENERGY CAPTRADES
  HOLDING CORP.

  
	
   

  	
  RELIANT ENERGY
  SABINE (DELAWARE), INC.

  
	
   

  	
   

  
	
   

  	
   

  
	
   

  	
  By: 

  	
  /s/ Patricia F.
  Genzel

  
	
   

  	
  Name:

  	
  Patricia F.
  Genzel

  
	
   

  	
  Title:

  	
  President

  
					



 



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  J.P. MORGAN TRUST COMPANY, NATIONAL ASSOCIATION, as
  Trustee

  
	
   

  	
   

  
	
   

  	
   

  
	
   

  	
  By: 

  	
   /s/ Michael
  J. Judge

  	
   

  
	
   

  	
   

  	
  Authorized
  Signatory

  	
   

  







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