This paper presents a power generation and heat recovery model for reciprocating internal combustion engines (ICEs) that can be effectively used in simulations of combined heat and power (CHP) systems. Reciprocating engines are among the most common types of power generation units in CHP systems. In the literature, constant engine efficiencies or empirical efficiency curves are commonly used in the simulations for CHP performance evaluation. These methods do not provide realistic results for the design and component selection processes. The main advantage of this model is that it provides estimates of performance/efficiency maps for both electrical power output and useful thermal output for various capacities of engines without experimental data. The intent of this model is to provide performance/efficiency maps during a preliminary CHP design/simulation process. An example of model calibration to a specific CHP application is presented to demonstrate the capability and benefit of this model. The simulation results are validated with manufacturer’s technical data.