Diesel is the main fuel for use in industrial, transport, and in diesel power plants; unfortunately, it is polluting and non-renewable. This study proposes conversion of a 120 MW diesel engine power plant is converted to a natural gas-powered plant with the objective of reducing emissions and cost of power by avoiding and reducing generation from heavy fuel oil. The study showed that the use of natural gas simultaneously reduces the cost of generation per kWh and emissions, particularly SO_X. The conversion process involves modification of the fuel injection system to cope with new fuel injection and ignition requirements, new control for fuel injection and reduced air to fuel ratio and lowering of the engines’ compression ratios through increase of the engine cylinder clearance volume. These fuel savings will influence renegotiation of the power purchase agreement resulting into lower tariffs, higher load factor and power plant utilization which translates to more revenue and reduced unit cost of power with lower environmental impact as well as better return on investment from the largest grid connected diesel power plant in East and Central Africa. This study showed that dual fuel diesel power plants running on both natural gas and diesel exhibit better performance indicators in terms of engine specific fuel consumption, engine brake thermal efficiency, diesel engine indicated thermal efficiency, the power plant electricity generating unit cost of power, the total engine emissions and hence leading to less environmental impact. Therefore, the engine fuel conversion will reduce the cost of power and emissions.