In a mass burn waste incinerator, the waste undergoes combustion in the bed on the grate and the products of incomplete combustion are destroyed in the gas plenum above the bed. Thus, it is essential to consider the reaction in both the waste bed and the gas plenum when evaluating the combustion performance using computational fluid dynamics (CFD). However, solving the two-phase reacting bed of waste is not straightforward in CFD simulations. This paper presents the combustion and gas flow of a waste incinerator using a new simulation approach that incorporates two models: FLIC for the reacting bed of solid fuels and FLUENT for the turbulent reacting flow of the gas plenum. The two models were linked through the boundary conditions on the waste bed in such a manner that FLIC provides the gas properties leaving the bed as an inlet condition of the gas flow and FLUENT produces the radiation transferred to the bed. The predicted results were compared with on-site measurements in the plant by a novel instrument that collected the temperature, oxygen concentration, and motion data while tumbling with the waste particles within the bed. The measured data were highly fluctuating, but FLIC satisfactorily predicted the overall trend of temperature and oxygen concentration including the upper and lower boundaries of the violent fluctuations. The FLIC/FLUENT combined simulation provided crucial information on the nature of combustion and flow characteristics such as the ignition and burnout points of waste, the rates of each combustion process, and the subsequent gas flow pattern in the combustion chamber.