Thermodynamics plays an important role in performing energy and exergy analyses of industrial processes. The first law is widely used in engineering practice, and is the basis of the heat-balance method of analysis that is commonly used in energy systems performance analysis. However, the second law involves the reversibility or irreversibility of processes, and is a very important aspect of the exergy method of energy systems analysis. For a sustainable future, exergy analysis allows for improvements not necessarily attainable via energy methods, like increased efficiency, reduced fuel use and reduced environmental emissions. From the viewpoints of energy conservation and environmental benefits, cogeneration systems can be considered as systems of sustainable energies. From this point of view, in this study, the energy, exergy and economic analysis of an actual natural gas cogeneration plant with a total capacity of 121.5 MWh electrical powers is carried out. The first and second law efficiency of the cogeneration plant is calculated as 40.8% and 90.2%, respectively. The pay-back period of the cogeneration plant is calculated as three years, which means this plant runs quite efficiency and economically.