Underground coal mining operations produce large numbers of gobs that contain methane from mining coal seams and their adjacent coal seams. In longwall mining, gob methane is released to coal faces, presenting safety issues. Therefore, if gob methane sources can by identified accurately under extraction conditions, then efficient methane extraction strategies can be adopted to maximize the capture of methane from gobs and to control effectively methane flow to the coal face, thereby reducing safety risks. In this study, a new method to determine quantitatively gob methane sources under extraction conditions was proposed and tested in China’s Liyazhuang Coal Mine. Firstly, the isotope and gas composition of methane desorbed from coal cores of the mining seams and their adjacent seams, as well as methane collected from methane drainage boreholes at different heights in the gobs, were measured. Then, an isotope source model for calculating gob methane sources was established. Finally, based on isotope and gas compositions, the proportions of sources of gob methane that had been extracted from methane drainage boreholes were determined using the afore-mentioned model, and the extraction range that could maximize efficiently the control of methane emissions from each methane source was obtained.