Numerical methods are used to model, predict and optimize energy systems in which thermodynamic analysis is coupled with design and economic evaluation. Two examples, in which the cost objective functions are assumed, are presented. The first example is a regenerative gas-turbine cycle in which several thermodynamic and design decision parameters are optimized for the given structure are considered. The second is a steam bottoming cycle, in which structural as well as parametric changes are introduced. Structural changes assume generating steam at several pressures and parametric changes assume different pressures, pinch and terminal temperature differences and different sets of pressure losses. The essential features of a software adequate for this type of analysis are described.