An algorithm is developed for the rapid estimation of methane production rate in anaerobic digesters. The algorithm includes a detailed biochemical reactions scheme, that makes it capable to cope with variations in the feed composition, as usually happens in co-digestion plants. It consists of a system of nonlinear algebraic equations that can be easily solved in a sequential way. The algorithm is evaluated based on previously published results but also on production data from a pilot-plant that co-digests cheese whey with poultry manure. Last, the algorithm is compared with other suggested simplified model used to the same aim, and applied to study further the co-digestion of the above combination of waste.