The entrained flow reactor (EFR) is frequently applied to analyze the reactivity of pulverized coal char under controlled atmospheres of temperature and partial pressure. The rate constants for a specific char gasification model derived from EFR tests may be subject to errors due to the differences between the target and actual conditions caused by (i) the shorter reaction time because of the thermal entry region if not preheated to the target temperature, (ii) lower particle temperatures because of endothermic reactions, and (iii) lower partial pressure of reactants consumed. In this study, coal char was gasified using CO2 and H2O in a pressurized EFR, and the validity of the rate constants for a char conversion model based on the target reaction conditions was assessed using computational fluid dynamics (CFD). Then, an analytical method was proposed to determine the effective reaction time, particle temperature, and reactant partial pressure for derivation of rate constants only using the experimental data. The predicted char conversion ratios adopting the modified rate constants showed good agreement with the measured data under various conditions. The method can be applied to deriving rate constants for a particular char conversion model and to estimating the uncertainties of a particular EFR design and test conditions.