The thermoeconomic optimization of an actual steam power plant coupled with a MSF desalination unit is reported. A global optimization of the whole system is performed based on separated local optimizations of different plant units. The local optimization procedure described herein requires fewer computing resources and deals with simpler mathematical problems than conventional optimization methods. On the other hand, the local optimization method requires a thermoeconomic model providing the exergy and economic costs of all mass and energy flows of a plant, including those corresponding to fresh water and electricity produced. This application can be very useful, either for the plant management in order to achieve a cost-effective operation, and for a better plant design. In the example given, approximately 11% of the total cost was saved according to the optimization results in the nominal operating conditions of the plant.