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-productgas-fired GTCC is 6% lower than that of LNG-fired GTCC at a combustion temperature of 1573 K. The generating efficiency of the gas-separation energy was improved by +4.4% (eta = 48.8 -> 53.2%) by removing CO2 and N-2 from the blast-furnace gas. If this innovative system were applied to steelworks around the world, the potential electricity generation would be 55 billion kWh/year. This would mean a reduction of approximately 45 million t-CO2/year.