Supercritical water fluidized bed (SCWFB) gasifier is a promising reactor for gasification of coal in supercritical water. For the lack of study on radiation property of supercritical water, former researchers all ignore the radiative heat exchange in numerical modelling of reactor and consequently obtain poor accurate simulation results. To make up for the research deficiencies, firstly, this paper applies the line-by-line (LBL) calculations to explore the radiative characteristics of supercritical water and presents the Planck-mean absorption coefficient of water under pressure of 23 MPa. Secondly, employing the calculated absorption coefficient, a comprehensive computational fluid hydrodynamics (CFD) model with radiation heat transfer, fluid-particle two-phase flow and chemical reaction kinetics considered is developed to simulate the coal gasification in a SCWFB reactor. In this model, the gas and particle radiation are solved using discrete ordinate (DO) model, fluid-particle flow is modeled by dense discrete phase model (DDPM), and gasification reaction is investigated with finite-rate model. Simulation results show good agreement with experimental data, the average absolute deviation between predicted and measured temperature is less than 16K (1.9%), which verifies the accuracy of the current numerical model. Results indicate that thermal radiation approximately accounts for 50% of total heat transfer, and has a significant impact on the temperature distribution as well as prediction of gasification process. This study reveals the characteristics of heat transfer in the reactor, and the CFD model developed in this paper provides a convenient way for optimizing reactor design.