A constrained thermodynamic model was developed in order to describe the product gas composition of a methanation process. The non-equilibrium affinities of CO2 and CO methanation reactions were applied as additional constraints to the chemical system while solving the local thermodynamic equilibrium. The affinities of these reactions were modelled as a linear temperature dependent function based on published and experimental data. The applicability of modelling methodology was validated against two independent literature datasets and against our own experiments. In all cases, the model predicted the product gas composition of main species CO, CO2, H-2, H2O and CH4. In two cases, it was sufficient to add only a single constraint describing the affinity of CO2 methanation reaction and in one case, two constraints describing the affinities of CO and CO2 reactions were needed. Results suggest that the modelling methodology presented can be utilised to describe different kinds of methanation process conditions, but the affinity models need to be defined based on the experimental reaction kinetic data. This methodology and the developed 0D-reactor model is applicable, for example, as part of a larger process simulation where additional details of methanation process are needed. (C) 2017 Elsevier Ltd. All rights reserved.