The invention relates to a method for producing a high-molecular polycondensate from a solidified polycondensate prepolymer by solid phase polycondensation. According to said method, the polycondensation cleavage products of the solid phase polycondensation reaction are extracted from the product by means of a process gas, and the process gas is then cleaned of the polycondensation cleavage products and essentially recycled. According to the invention, the specific energy input is less than 120 kWh/t, preferably less than 110 kWh/t and especially between 70 and 100 kWh/t. The invention also relates to an installation for producing a high-molecular polycondensate, said installation comprising a heating appliance (1), followed by a crystallisation appliance (2), a reaction appliance (4), and a cooling appliance (5). A gas outlet (5d) of the cooling appliance (5) is connected to a gas inlet (1c) of the heating appliance (1), a gas outlet (4d) of the reaction appliance (4) is connected to a gas inlet (6c) in a gas cleaning system (6), and a gas outlet (6d) of the gas cleaning system (6) is connected to a gas inlet (4c) of the reaction appliance (4).