The electricity demand is ever increasing, and the worldwide power generation is largely dependent on the rapidly dwindling fossil fuel resources. It is no different in Bangladesh as well where the prevailing dynamic in the energy sector projects to a certain lack of sustainability. This makes the performance evaluation to determine and improve the efficiency of energy conversion systems an unavoidable requisite. This study investigates the energetic and exergetic performance of a gas engine-based (4 MW) combined heat and power system situated at Rajshahi District in Bangladesh. Different thermodynamic performance parameters along with the exergo-environmental effect and the sustainability of the system are also assessed additionally. The overall system and individual components are analyzed based on the actual data measured on site at 48% engine load and a constant engine speed of 1500 rpm. The results reveal that the maximum energy loss and exergy destruction occur in the reciprocating engine with values of 656.54 kW and 1835.04 kW, respectively, followed by the heat recovery steam generator with 465.51 kW energy loss and 671.51 kW exergy destruction.The overall system has energy and exergy efficiency of 29.95% and 23.12%, respectively. The exergetic sustainability index of the system is found to be 1.30.