There have been reports of North American municipal solid waste landfills exhibiting temperatures in excess of 80 °C. Although mathematical models have been developed to predict heat generation and accumulation in landfills, predictions have not been compared to temperature data from a full-scale landfill that receives heat generating ash. The objectives of this study were to apply a three-dimensional finite element model to a southeastern U.S. landfill and to compare model predictions with field data. The model incorporates gas–liquid–heat reactive transfer with exothermic biological reactions and hydration and carbonation of ash. An 8-step reconstruction approach digitalized the landfill geometry for the incorporation of a site-specific waste disposal strategy and initial and boundary conditions. The model was calibrated to adjust laboratory-measured rates of ash hydration and carbonation to the field rates. Once calibrated, the results showed a total root-mean-square error of 11 °C across 40 measurements in five temperature probes. The model predicted an elevated temperature zone in a region of the landfill between two temperature probes, and the predicted temperatures were consistent with the temperature trends in gas collection wells. The model is sensitive to the CaO content of ash, highlighting the importance of understanding the ash composition prior to disposal.