Coke oven gas (COG) is one of the main by-products generated from steel refineries. In most cases, COG is used only to generate low pressure steam for process heat needs or for small amounts of electricity production. Carbon emissions are inevitable in this COG utilization method. However, carbon emissions are no longer free: the federal government has proposed a minimum carbon tax scheme which incentivises the steel industry to upgrade their present system in order to maintain their profitability. Based on a local steel refinery case study, we propose retrofitting the plant with a combined cycle power plant (CCPP) using COG as fuel input in order to help reduce CO2 emissions and increase energy efficiency. However, COG contains a significant amount of sulphur compounds which have to be removed before it can be used as fuel input for CCPP. Therefore, a MDEA based desulphurization process is proposed to reduce the main sulphur content (H2S) in COG to less than 1 ppmv. ProMax is used to simulate this acid gas removal process. The balance of the proposed CCPP plant is developed and simulated in Aspen Plus. Under the uncertainty of electricity price, the CCPP plant is further optimized using a surrogate model in GAMS to global optimality with net present value (NPV) as the objective function. The optimized CCPP plant was observed to generate more than twice the electricity compared to the status quo of the existing steel refinery. By upgrading the present system to CCPP, the local steel refinery can expect an additional NPV of up to $98 million with $68.5 million in capital investment. At the same time, an indirect 84.1 g of CO2 emission is reduced for every 1 kg of COG consumed by displacing Ontario grid electricity. The benefits are even higher if this retrofit could be applied to steel refineries elsewhere in the world with more carbon-intensive electricity grids. As such, if this upgrade could be implemented on all steel refineries in the world, the net lifecycle CO2 emissions of the steelmaking industry as a whole could be reduced by as much as 108 MtCO2e/yr.