In this paper, the application of an iterative exergoeconomic methodology for improvement of thermal systems to a complex combined-cycle cogeneration plant is presented. The methodology integrates exergoeconomics with a professional process simulator, and represents an alternative to conventional mathematical optimization techniques, because it reduces substantially the number of variables to be considered in the improvement process. By exploiting the computational power of a simulator, the integrated approach permits the optimization routine to ignore the variables associated with the thermodynamic equations, and thus to deal only with the economic equations and objective function. In addition, the methodology combines recent available exergoeconomic techniques with qualitative and quantitative criteria to identify only those decision variables, which matter for the improvement of the system. To demonstrate the strengths of the methodology, it is here applied to a 24-component cogeneration plant, which requires 0(103) variables for its simulation. The results which are obtained, are compared to those reached using a conventional mathematical optimization procedure, also coupled to the process simulator. It is shown that, for engineering purposes, improvement of the system is often more cost effective and less time consuming than optimization of the system. (C) 2009 Elsevier Ltd. All rights reserved.