NOVELTY - Producing biogas from lignocellulosic biomass, preferably straw, comprises: (a) mixing the lignocellulosic biomass with aqueous solutions at a temperature of 60-100 degrees C, preferably 70-85 degrees C, with the process water and/or by using of steam; (b) heating the lignocellulosic biomass at 130-200 degrees C, preferably 150-180 degrees C, and allowing the heated lignocellulosic biomass to stand for a period of 5-120 minutes, preferably 10-30 minutes; (c) carrying out anaerobic fermentation of the thermally treated lignocellulosic biomass using microorganisms and recovering the biogas produced.    USE - The method is useful for producing biogas from lignocellulosic biomass, preferably straw (claimed).    ADVANTAGE - The method: is highly energy- and resource-efficient and cost-efficient; and reduces the carbon dioxide emissions.    DETAILED DESCRIPTION - Producing biogas from lignocellulosic biomass, preferably straw, comprises: (a) mixing the lignocellulosic biomass having an average particle size of less than 200 mm, preferably less than 20 mm, with aqueous solutions at a temperature of 60-100 degrees C, preferably 70-85 degrees C, with the process water and/or by using of steam; (b) heating the lignocellulosic biomass at 130-200 degrees C, preferably 150-180 degrees C, and allowing the heated lignocellulosic biomass to stand for a period of 5-120 minutes, preferably 10-30 minutes, preferably in a continuous-flow reactor chamber for thermal decomposition; (c) carrying out anaerobic fermentation of the thermally treated lignocellulosic biomass using microorganisms and recovering the biogas produced. An INDEPENDENT CLAIM is also included for a system comprising a conveyor for the transport of lignocellulosic biomass, a damper or mixer for mixing the lignocellulosic biomass with water, at least one digester for a single or multi-stage anaerobic fermentation, a phase separator for at least proportionate recovery of aqueous phase from the end of the fermentation, a conveyor for recycling the recovered aqueous phase in the process, a thermal decomposition reactor optionally to thermally unlock along with a heater, which has at least an anaerobic digester; and a conveyor for the transporting heat-treated lignocellulosic biomass into an anaerobic fermenter.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic process flow of the method for producing biogas from lignocellulosic biomass (Drawing includes non-English language text).    Dry grinding (1)    First admixing (2)    Second admixing (3)    Wet grinding (4)    Heat exchanger (5)