In this paper, a complex thermal plant is analyzed. The plant under analysis is basically made up of four gas-fueled engines with heat recovery. Each engine can drive, simultaneously, an electric generator and the compressor of a heat-pump/chiller. The plant is interconnected to the electric utility grid, both to receive additional power and to deliver excess power. In addition, each heat-pump/chiller can be driven electrically, using the electric generator as a motor. For any given amount of power and heat required by the users, a large number of operating conditions are possible. So, the problem arises of selecting, at any given time, the operational mode that involves the lowest instantaneous cost of operating the system. This cost can be regarded as the objective function to be minimized in a typical constrained optimization problem. In the paper, after a short description of the plant, this problem is dealt with. Some examples of the results provided by the optimization algorithm are presented and commented on. The economic savings achievable through an optimized operation are highlighted too.This paper was presented at the ECOS'00 Conference in Enschede, July 5-7, 2000