The water-cooled wall of the gasifier plays an important role in maintaining the safe and stable operation of the gasifier under high temperature and pressure. The ANSYS software was used to simulate the temperature distribution on the inner wall surface of the water-cooled wall. The effects of three conditions including slag nails, no slag nails, and replacement of slag nails by gills of equal area on the heat transfer process were investigated. The influence of the arrangement of nails on the temperature of water-cooled wall was researched. The results showed that the slag nails can effectively improve the heat transfer efficiency and take away the excess heat in the gasifier in time. The slag nails with a small angle and a triangular arrangement had better heat transfer efficiency and more uniform temperature distribution. This simulation calculation was significant for the design of heat transfer structure and long-term safe operation of water-cooled wall gasifier.