Abstract: The entrained-flow coal gasifier coupled with a radiant syngas cooler (RSC) can recover the sensible heat of high-temperature syngas (above approximately 800 °C) in the form of by-product 11 MPa high-pressure saturated steam; understanding its flow field and ash deposition characteristics is crucial for the optimized design of the RSC. Compared with the WSGGM model for radiative heat transfer in high-temperature ash-laden gases within coal-fired boilers, this study employs the modified WSGGM-G model to conduct numerical simulations of flow and heat transfer within the RSC. This is due to the significant differences in the radiative properties of H₂O, CO₂ and CO gases within the high-pressure gasifier compared to those in the boiler. The results indicate that when the total volume fraction of absorbed gases (H₂O and CO₂) within the RSC is below 10%, the heat recovery efficiency is more significantly affected by variations in the concentration of absorbed gases, whilst the contribution of radiation heat transfer from ash particles accounts for only 2.5% of the steam production; ash accumulation is most severe in the region near the tail of the jet at the RSC inlet, reaching equilibrium after 20 d of operation, at which point steam production decreases by more than 30%. Suppressing the backflow of ash-laden synthesis gas within the RSC and reducing the extent of ash accumulation in the region near the jet tail are key factors in improving the heat transfer efficiency of the RSC.