In this work, the authors have studied a method for the improvement of the generating efficiency of a gas-turbine combined-cycle system (GTCC) using by-product gas (pretreated blast-furnace gas). The generating efficiency of by-product-gas-fired GTCC is 7 % lower than that of LNG-fired GTCC at a combustion temperature of 1623 K. The generating efficiency of the gas-separation energy was improved by +5.5 % (η=48.8→54.3 %) by removing CO₂ and N₂ from the blast-furnace gas. If this innovative system were applied to steelworks around the world, the potential electricity generation would be 69 billion kWh/year. This would mean a reduction of Δ56 million t-CO₂/year.