In this work, we are interested in the estimation of CO2 and H2S heats of absorption in aqueous solutions of alkanolamine: monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA). Two methods can be used to calculate the heat release during the absorption phenomenon. The easier which consists of applying the integration of the Gibbs-Helmholtz expression remains inaccurate. The second one, more rigorous, evaluates the heat transfer through an internal energy balance for an open system. The balance expression contains partial molar enthalpies of species in the liquid phase which are calculated from the electrolyte nonrandom-two-liquid (NRTL) excess Gibbs energy model. The calculations carried out in this method can be considered as predictive regarding the NRTL model because its interaction parameters were previously and solely fitted on vapor-liquid equilibrium (VLE) data and not on experimental heat of absorption data. The comparison between both methods and experimental data for the three alkanolamines shows the contribution of this rigorous calculation to better estimate both properties (i.e., solubility and heat) and its usefulness to improve processes. Heats of absorption calculated with the second method can be used in addition to VLE data to fit NRTL parameters. This procedure leads to less-correlated parameters and allows extrapolating the model with more confidence.