The effect of CO on the gasification of a Polish coal-derived char was investigated in a fluidised bed from 1123 to 1248 K. Rate expressions developed from Ergun’s mechanism or a modified three-step reaction mechanism, coupled with Cylindrical Pore Interpolation Model (CPIM) to account for the intra-particle mass transfer, were developed to predict gasification and the effect of CO. Compared to the Ergun’s rate expression, the three-step expression has an extra term for pco, making the inhibition effect of CO more pronounced. The agreement between experimental and numerical results was satisfactory for both models simulating gasification by CO2/N2. Then, when CO (1% or 3%) was intentionally introduced in the feed gas (CO2/N2), the gasification rates significantly decreased. It was found that the model based on Ergun’s mechanism over-estimated the gasification rate, while the results from the model with the three-step mechanism agreed with the experimental data.