Given continuously increasing global energy needs, diversified efforts have been made to find and exploit new natural gas resources. These include coalbed methane (CBM), which represents an important global, unconventional source of natural gas. Efforts have been underway for some time to more effectively generate methane in-situ in coal plays by introduction of nutrients and/or microbial consortia. However, much is still to be learned about the limitations and environmental conditions that support microbial growth and are conducive to biogenic methane production from coal. The present investigation evaluated environmental conditions that led to increased methane production from subbituminous coal by introducing a foreign methanogenic consortium that included Methanobacterium sp. A central composite design (CCD) was used to explore a broad range of operational conditions, examine the effects of the important environmental factors, such as temperature, pH and salt concentration, and query a feasible region of operation to maximize methane production from coal. An anticipated detrimental effect of NaCl concentration on methane production was observed for the consortium assessed. The range of feasible operational conditions comprised initial pH values between 4.2 and 6.8, temperatures between 23 degrees C and 37 degrees C, and NaCl concentrations between 3.7 mg/cm(3) and 9.0 mg/cm(3). Coal biogasification was optimal for this consortium at an initial pH value of 5.5, at 30 degrees C, and at a NaCl concentration 3.7 mg/cm(3) (i.e., 145,165 ppm, which is 25.6 sft(3)/ton).