Among the various aspects that have to be analysed in a cogeneration and combined cycle plant design, the exhaust gas stack design can represent a critical aspect, in particular when a by-pass stack, which allows the modulation of heat-to-power generation, is present, since it may influence the entire system working condition. To properly take into account the large number of the different requirements which enter in an exhaust gas system design, a multidisciplinary analysis involving numerical integrated approaches can be adopted in order to obtain an optimally designed stack system. In this paper, the design of the exhaust gas system in a cogeneration plant is analysed. The design is performed numerically through a three-dimensional integrated numerical code. Different design solutions are simulated and the results discussed in detail. (C) 2008 Elsevier Ltd. All rights reserved.