The development of district heating systems in Poland up to 1990 is characterized with particular reference to the combined generation of heat and electrical energy in fossil-fired and nuclear heat-and-power plants. The proposed method of an economic evaluation of nuclear heat-and-power schemes consists in comparing them with the equivalent fossil-fired plants and minimizing the total annual costs of heat energy generation. Hence, the optimum value of the so-called combination factor α can be obtained, this factor being defined as the ratio of heat output at the steam turbine outlets to the maximum heating load of the whole heat-and-power plant. As an example of the optimization procedure, three particular values of the maximum heating load Qmax in given areas are considered: Qmax = 1000, 1500, and 2000 MW, and in each case the optimum a values are determined.