In contrast to most of the other computer codes which employ the LMTD method, the Ε - NTU method is used in this code for sizing and for predicting the off-design performance of multi-pressure level HRSG (Heat Recovery Steam Generator) systems. The Ε-NTU approach has some advantages particularly in a performance calculation over the LMTD method. The Ε-NTU method is quite simple and straight-forward while the LMTD method involves successive approximation. Improvements in calculation stability, rate of convergence and accuracy of solution are discussed. The basic equations and solution strategy are presented.