A mathematical model of terrestrial biomass biogasification is presented. This model calculates mass and energy balances and the levelized cost of synthetic natural gas (SNG). It consists of interconnected modules for harvesting, biomass transportation, biogasification, gas processing, energy balance, global climate change, and economics. The conversion module incorporates bioconversion technology recently developed at pilot scale at Cornell University and Walt Disney World. In large-scale biomass biogasification, in any climate at any reactor temperature, process heat needs are small and could be reduced to zero if metabolic heat production is included. Therefore, thermophilic digestion should be preferred to mesophilic, to the extent that the former results in higher reaction rates. Preliminary sensitivity analyses were conducted. Economies of scale are described. Increases in biomass productivity yield diminishing returns above 45-55 Mg/ha year (20-25 dry tons/ac). Distribution of fields around the conversion facility is unimportant as long as at least thirty percent of the surrounding area is planted in energy crops. Improvements in methane yield result in continuing returns, confirming the importance of this parameter. Only a minor penalty is incurred for overdesigning the retention time up to fifty percent; such overdesign would greatly enhance reliability. The research state-of-the-art is slightly above $6/GJ (MMBtu). Sale of coproducts could reduce this cost in half. The potential impact of energy crops on global climate change is quantified.