The Allam cycle is a thermodynamic cycle for the combustion of gaseous fuels under oxyfuel conditions with inherent carbon capture. As the CO2 is captured intrinsically, the efficiency penalty of capture on the overall plant is small, meaning that Allam cycle power plants achieve a similar efficiency to traditional fossil fuel power plants without carbon capture and storage. At high-pressures and a high-CO2 dilution, combustion mechanisms are poorly understood. Sensitivity and quantitative analysis of four established chemical kinetic mechanisms were used to determine important reactions and the best performing mechanisms at different conditions. CH3O2 chemistry was identified as a pivotal mechanism component for modelling methane combustion above 200 atm. The University of Sheffield (UoS) supercritical CO2 (sCO2) mechanism created in the present work better models the ignition delay time (IDT) of high-pressure combustion in a large dilution of CO2. Quantitative analysis showed that the UoS sCO2 mechanism was the best fit to the greatest number of IDT datasets and had the lowest average absolute error value, indicating the superior performance compared to four existing chemical kinetic mechanisms, well-validated for lower pressure conditions.