Abstract The paper quantifies the synergy-effects of an areal combination of biogas-plants with plants of the building materials industry (e.g. cement works) from the energetic and economical point of view. Therefore an overall process model based on energy and mass flow balances is developed to determine the effects of a combination of both plants in terms of energetic efficiency, investment and operating costs, greenhouse gas emission reduction and overall energy production costs. The results and the calculation procedure for a combination of biogas plants with cement works are presented in detail. The main benefits of this combination are the utilisation of low temperature excess heat sources from cement works for fermenter heating and the direct thermal utilisation of unprocessed biogas as a valuable, CO2-neutral fuel for combustion processes for instance clinker burning. Due to the combination, the energetic efficiency of the biogas plant, defined as utilisable energy output in relation to the energy content of the produced biogas, significantly increases from 63.0% to 83.8%. Concurrently the energy production costs are reduced, turning biogas into a competitive source of energy without the need for federal sponsorship. Calculations show, that production costs in combined plants for plant sizes larger than 90 m³STP/h biogas are even lower than the actual market prize of natural gas.