TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The refinery unit of the BtL plant is integrated in a fractionating distillation column (12), where the tall oil is separated substantially into two fractionated streams of tall oil so that the lighter fraction is processed further into hydrocarbon chains suited for producing high-quality biofuels while the heavier bottoms fraction (15) is passed to the gasification process and/or energy generation of the BtL plant. The tall oil distillate separated in the fractionating distillation column is passed to a clarifier, where the separated the fraction is to be refined and has a composition similar to the heavy Fischer-Tropsch liquid (HFTL) fraction produced in an Fischer-Tropsch reactor, while the remainder of the lighter fractions is utilized in energy or hydrogen generation. The lighter fractions of the tall oil distillate having a composition similar to the HFTL fraction produced in an Fischer-Tropsch reactor are pretreated prior to separation in the fractionating distillation column, and after the separation step the lighter fractions are passed to a refinery unit and further passed to a hydrocracker. The heavier tall oil bottoms fraction received from the distillation column is pumped to the gasification reactor of the BtL plant, where a high-temperature gasification reactor and/or, when necessary, is used in energy generation or is returned for combustion in the lime kiln of the cellulose plant. The high-temperature gasification reactor is a pressurized oxygen gasifier which is fed with the tall oil bottoms fraction that joins with the gas resulting from the treatment of biomass in the reactor burner and reacts with oxygen thus generating raw syngas. In the method depending on the amount of tall oil feedstock, the product yield of the BtL plant increases by 5-50%. The BtL plant is integrated with a cellulose plant, where the BtL plant utilizes tall oil obtained from the plant integrated with it and, when necessary, from other cellulose plants for producing biofuel and utilizing the biofuel in the gasification step of the BtL process and/or in energy generation. The tall oil is separated prior to the gasification step of the BtL plant in a fractionating distillation column into a light tall oil fraction that is processed together with HFTL fraction obtained from the BtL process. The tall oil is preprocessed, refined and passed to a hydrocracker for producing a tall oil fraction that is passed forward as a side-stream to the final distillation step of the BtL product for producing a biofuel. The heavier bottoms fraction obtained from the distillation column is utilized in the gasification process and/or energy generation of the BtL plant or in the lime kiln of the cellulose plant, where the BtL plant is integrated. EXAMPLE - No suitable example given.