This paper addresses the problem of automatically synthesize heat exchanger network that features minimum global cost. We present a new approach including HEN configuration choice by simulated annealing algorithm and NLP formulation to systematically optimize their operating conditions (temperatures and split rates). Particular attention is made to the size reduction of the NLP problem which has been the subject of a precedent paper (Athier et al., 1996). In this paper, the global methodology used to solve this problem is presented. The proposed approach is illustrated by two examples, the 5SP1 problem with forbidden matches and a more large scale problem, the 14SP1 one.