This study aimed to assess the thermodynamic performance of an Organic Rankine Cycle (ORC) unit for power generation operated by the industrial waste heat of exhaust gas of a steam power plant. For conducting exergoeconomic and environmental analyses, real data measured from a coal-fired power plant operating under subcritical conditions at the highest capacity of 660 MW was utilized. The main goal of this study is to increase the performance of the subcritical power plant to reach the performance of the supercritical steam power plant by converting waste heat into electricity. As a working fluid, R600, R245fa, R236ea and R236fa working fluids were chosen for the present system. The influences of flue gas temperature, evaporator pinch point temperature and the steam power plant unit load on the power plant performance were determined. According to the results obtained, ORC exergy efficiency reached the highest value of 42.26%, and a net power output of 4.7 MW was achieved, thereby rising overall exergy efficiency by 0.3%. Calculations illustrate that the exergy efficiency enhances with a steam power plant unit load. Conversely, this exergy efficiency decreases with an increase in flue gas temperature and evaporator pinch point temperature. In addition, the power output generated by the ORC unit is increased with increasing flue gas temperature and steam power plant unit load. But, this net power output is reduced with rising the evaporator pinch point temperature. The novelty of this work is the flue gas of steam power station can confirm to be a suitable source of low-temperature heat for conversion into electrical energy, thereby enhancing the general performance of the process and lowering environmental impact.