This paper refers to recovering waste heat from the hot gases exhausted by internal combustion engines (ICEs) driving electric generators (EGs) at full and partial load. The topic is of particular interest for developing countries where electric grids are underdeveloped or missing and electricity is generated locally by using classical fuels. The heat recovery system is based on an Organic Rankine Cycle (ORC). A novel method is proposed for the optimum design of ORC-based systems operating in combination with ICE at partial EG loads. First, ORC-based systems coupled with ICEs operating at full EG load is treated. Specific results for the operation at full EG load are as follows: (i) the optimum superheating increment ranges between 30 and 40 degrees C, depending on the type of the working fluids; (ii) a pinch point temperature difference exits between the flue gas temperature and the working fluid at the evaporator inlet; (iii) the total area of the evaporator is very close to the total area of the condenser, a fact which facilitates manufacturing; (iv) the surface area of the preheater zone is about 75% of the total surface area, while those of the boiler zone and superheater zone is about 13.5% and 11.5%, respectively. Second, the case of the ORC-based systems coupled with ICEs operating at partial EG load is considered. Specific results for this case are as follows: (v) the net power may be maximized by optimizing the working fluid mass flow rate; (vi) when the ICE is coupled with an ORC-based system, the overall thermal efficiency of the combined system, eta(ICE-ORC), is higher than the thermal efficiency of the ICE operating alone. As an example, for the case treated here, 'eta(ICE-ORC) is higher than Hits. by 6.00%, 5.85% and 5.91%, for EG loads of 100%, 75% and 50%, respectively. (C) 2017 Elsevier Ltd.