Display omitted •An ex-situ experiment of coal gasification under high pressures and temperatures.•Oxygen and steam injected under different molar ratios used as gasification agents.•A comparison between semi-anthracite and bituminous coals for syngas production.•Increase in pressure and the amount of steam benefit methane-rich syngas production.•Gasification temperature of 750 °C is the most favourable for methane generation. The results of ex-situ small-scale laboratory tests performed in a bespoke batch reactor simulating coal gasification to find the most optimal experimental conditions for producing methane-rich syngas in the context of UCG are presented in this paper. The influence of gaseous reactants (oxygen and steam), their supply rates and thermodynamic conditions (temperatures of 650 °C, 750 °C, 850 °C and pressures of 20 bar and 36 bar) on the gasification of semi-anthracite (South Wales coalfield) and bituminous (Silesian basin) coals is investigated. Increasing the gasification pressure from 20 bar to 36 bar and doubling the amount of steam with respect to oxygen benefit the methane generation. Although temperature increase from 650 °C to 850 °C also benefits methane generation, gasification at 750 °C provides the most optimal conditions for methane-rich syngas production. Overall, the highest methane generation occurs at 750 °C, 36 bar and H2O:O2 = 2:1 yielding peak methane concentrations of 44.00 vol% and 35.55 vol%, and average methane concentrations of 15.34 vol% and 14.64 vol% for the semi-anthracite and bituminous coals, respectively. These findings demonstrate that an increase in coal rank favours the methane generation. Owing to high methane content, the syngas produced at such conditions contains the highest calorific value, although the generation of hydrogen and carbon monoxide is reduced in comparison to the experiments conducted at 850 °C. This study shows that gasification of bituminous and semi-anthracitic coals at elevated pressures can provide stable generation of methane-rich syngas whose quality can be controlled by the gasification temperature through the dynamics of steam and O2 supply rates.