The alternative fuels, such as biomass, municipal wastes, and underground coal gasification gas, become attractive to the power plants as renewable energy sources or economical fuels. However, the alternative fuels usually have much lower heating value and different chemical compositions from those of coal and natural gas. Firing these alternative fuels in the boilers that are originally designed for coal firing or natural gas firing may cause unexpected boiler operating issues and/or thermal performance degradation. A careful evaluation study is often required prior to implementation. This paper presents the results of a study that evaluated the feasibility of using an underground coal gasification gas as a reburn fuel. The evaluation was done on Eskom’s Majuba Unit 5, a 710 MWe opposed wall-fired boiler, located in South Africa. The study utilized heat transfer analysis and computational fluid dynamics models to (1) evaluate the impacts of firing low calorific fuel on boiler efficiency and the boiler auxiliary system performance, (2) develop a conceptual gas reburn injection system, and (3) evaluate the impacts of gas reburn on the boiler thermal performance and boiler emissions. The results indicate that the underground coal gasification gas can be an effective reburn fuel for the Majuba boiler with upgrades on the auxiliary systems.