This paper presents an application of a combined conceptual-rigorous approach by improving the energy performance of a large-scale chemical process. Conceptual design comprises the analysis of energy availability of the existing heat exchanger network (HEN) and its utility system by means of pinch analysis. A detailed synthesis of a modified HEN system is performed in the rigorous step by applying a mixed integer nonlinear programming (MINLP) optimization technique. A case study was treated by means of this methodology. Based on the results of the conceptual step some structural modifications of the process flowsheet have been suggested, from which the most important is to burn the exhausting purge gas and recover the energy of the flue gas back to the process through the HEN system. The final design has been obtained by simultaneous optimization of the HEN structure and parameters by mathematical programming. Improvement of the economic objective function of about 47% compared with the base flowsheet was achieved.