In this work, a fixed-bed gasifier was used to study the effects of some important parameters on both the temperature and the chemical composition of the off-gas produced in a CO2−O2coal gasification process. Also, the effect of these parameters on the gasification temperature was determined. The parameters tested included input-gas composition, coal rank, coal size and coal-bed depth. In addition, the reactivity of a sub-bituminous coal was investigated under different gasification temperatures. Gas chromatography was employed to analyse the chemical composition of the gases. The results indicated that the off-gas temperature increased with coal rank and decreased with both the CO2−O2ratio and the coal-bed depth. Generally, the amount of H2and CO in the off-gas decreased with both the coal rank and the CO2−O2ratio and increased with coal-bed depth. Coal size did not have a significant effect on the gasification process.