The RELAP5-3D system analysis code is a generic and flexible analysis program that is used for the analysis of steady-state and transient scenarios of nuclear reactors. While RELAP5-3D has been successfully applied to a very large range of problems, other analysis tools exist that provide either additional accuracy or capability. This paper describes an extensible integrated code system that utilizes the RELAP5-3D analysis code in conjunction with the R5EXEC executive. A description of the different types of time-step algorithms and data transfer methods is provided. Discussions of the various strengths and weaknesses of the coupled code system are provided. Finally, examples of how the coupled code system has been exercised are included.