A new superstructure rigorous model for mixed-integer nonlinear programming is proposed for optimal synthesis for large scale heat exchanger network without stream splitting (number of streams is over 50). The model is based on a new superstructure, comprises more feasible structures, and deals with the problem of "combination explosion" successfully. At the same time, a new improved genetic algorithm is proposed for solving the model. One industrial case of retrofit of Ethylene plant's HEN is reported as a case study.