This paper proposes an improved model to predict the performance of sewage sludge updraft gasification more accurately by using Aspen Plus. The reaction rate of gas-gas and gas-solid reactions as well as the formation and destruction of tar composition were not adequately accounted for in the previous models. To solve that, a pair of Continuous Stirred Tank Reactor and Plug Flow Reactor blocks was configured for reaction zones of the gasifier. The number of tar compositions inserted in the model was increased to 6 components including benzene, toluene, phenol, naphthalene, acid palmitic, and indene. In comparison to the previous model, the accuracy of syngas composition prediction increased by 10–20%, particularly, the error of prediction was below 40% for all compositions at an equivalence ratio of 0.15. To make the first comprehensive study on energy production from sewage sludge treatment in Malaysia, the validated gasification model was applied to develop an integrated waste-to-energy with gasification model for Pantai 1 sewage treatment plant. There was a higher energy generating efficiency for the steam generation of 15.5–16.5% than for electricity generation of 1.4–1.9%. The electricity demand for sewage sludge treatment was fully satisfied, while 5.4% of the steam demand was met.