Closure and post-closure periods in underground coal mines present specific risks that have to be handled with sound management practices in order to achieve sustainability within the mining sector. These risks may negatively affect the environment and result in hazards on the surface caused by phenomena occurring in the rock mass after mining operations. One of the hazards that has to be considered in the process of coal mine closure is gas, which is caused by methane emission after mining operations cease. This paper presents a forecast of methane emissions conducted within the framework of the Research Fund for Coal and Steel “MERIDA” project, using a model that was developed by the National Institute for the Environment and Industrial Hazards (INERIS) from France, and the Central Mining Institute (GIG) in Katowice, from Poland. This model enables the estimation of the volume of methane emitted into longwall goafs from relaxed undermined and overmined coal seams in order to assess in a further step the risk of methane emissions into the atmosphere from closed/sealed underground coal mines. For a critical analysis of the forecasted methane emissions into the longwall goafs, the results obtained with this model were compared with a gas decline curve generated for longwall goafs from closed/sealed underground coal mines in Australia, where long term full range data was available. The results of the analysis allowed the forecasted emissions and, thus, the accuracy of the model to be validated. The forecast was developed in the “Anna” coal mine, property of the PGG Company, which is located in the southern part of the Upper Silesian region in the south of Poland, near the border with the Czech Republic, and that is undergoing a closure process.