Cogeneration systems are important alternatives in reducing emissions due to the ability of generating both electricity and heat energy simultaneously from the same fuel. In this study, the reduction amounts of the most hazardous emissions such as CO2, NO, and SO2 with using different types of cogeneration systems have been analyzed. The main approach of this analysis determines the emission differences between cogeneration systems and separate power-heat systems in case of generating the same energy. In addition, the parameters affecting this emission reduction have been described. Consequently, limit conditions for emissions reduction have been determined and shown with a case study.