A new heat-balance model at a low air ratio of 0.2-0.5 has been developed. Prediction of bed temperature and gas temperature at the furnace outlet is important in designing a fluidized bed gasification furnace. Heat-balance models based on complete combustion are widely used to evaluate waste incineration plants. It is, however, impossible to apply these models to a fluidized bed gasification furnace because it is operated at low air ratio of 0.2-0.5. In this work, the effect of water content and air ratio on furnace temperature, gas composition and char yield were investigated using a 1 t/d-scale test facility. The bed and gas temperature were calculated to evaluate the validity of proposed model, By comparing calculated results with experimental data obtained in a 4.5 t/d test facility using another type of RDF, it was concluded that bed temperature and gas temperature at the furnace outlet could be predicted with satisfactory precision.