Researchers in related fields are concentrating on the use of metallurgical slag waste heat to deliver stable heat for the pyrolysis or gasification of carbon–based materials. The key technology to achieve the clean utilization of coal is coal gasification. An important challenge for the technology is how to increase the amount of H2 in the mixture while decreasing the energy consumption of the gasification process. The typical temperature of copper slag, a typical iron–rich metallurgical slag, is 1300 °C. Relevant studies have demonstrated that copper slag can increase H2 production and support the water gas shift (WGS) reaction in addition to providing stable heat for coal gasification. The largest contribution of copper slag to the catalytic effect of the WGS reaction is demonstrated by the density functional theory (DFT) calculations, which reveal that the presence of copper slag lowers the energy barrier of H2O molecules dissociation from 6.57 eV to 4.61 eV. Additionally, the reduction in the CO2 formation process on the copper slag surface from 4.92 eV to 3.94 eV encouraged CO2 formation. This explains how the WGS reaction, which is catalyzed by copper slag acting as a heat carrier during the pulverized coal–vapor gasification process, works.