Process for continuously preparing an isocyanate other than pentane 1,5-diisocyanate, by reacting the corresponding primary amine with phosgene in the presence of an inert substance, comprising the following steps: (i) providing a gas stream comprising the primary amine and having a temperature of 200°C to 430°C, preferably 250°C to 420°C, more preferably 250°C to 400°C; (ii) providing a phosgene-containing gas stream having a temperature of 200°C to 430°C, preferably 250°C to 420°C, more preferably 250°C to 400°C; (iii) providing a gaseous inert substance stream having a temperature of 100°C to 500°C, preferably 150°C to 450°C, more preferably 150°C to 400°C; (iv) feeding the streams 10, 20 and 30 from steps (i) to (iii) into a reactor for reaction of the primary amine with the phosgene in the gas phase, wherein a) the reactor has flow connection to an at least twin-wall tube comprising a first wall that encloses the inner portion of the tube and a second wall that forms a gap with the outside of the first wall; b) either the stream 10 or the stream 20 is guided through the inner portion of the at least twin-wall tube into the reactor; c) the stream 30 is guided through the gap 1300 between the first wall and the second wall of the at least twin-wall tube into the reactor; d) that stream 10 or 20 which is not guided through the inner portion of the twin-wall tube is guided past the outside of the second wall of the at least twin-wall tube such that it comes into contact with the stream 30 at the end of the at least twin-wall tube; (v) working up the gaseous crude product stream formed in the reactor to obtain the desired isocyanate, wherein the temperature of the gaseous phosgene-containing stream provided in step (ii) is adjusted to a value below the boiling point of the primary aromatic amine to be phosgenated under the conditions in step (iv). (i) providing a gas stream comprising the primary amine and having a temperature of 200°C to 430°C, preferably 250°C to 420°C, more preferably 250°C to 400°C; (ii) providing a phosgene-containing gas stream having a temperature of 200°C to 430°C, preferably 250°C to 420°C, more preferably 250°C to 400°C; (iii) providing a gaseous inert substance stream having a temperature of 100°C to 500°C, preferably 150°C to 450°C, more preferably 150°C to 400°C; (iv) feeding the streams 10, 20 and 30 from steps (i) to (iii) into a reactor for reaction of the primary amine with the phosgene in the gas phase, wherein a) the reactor has flow connection to an at least twin-wall tube comprising a first wall that encloses the inner portion of the tube and a second wall that forms a gap with the outside of the first wall; b) either the stream 10 or the stream 20 is guided through the inner portion of the at least twin-wall tube into the reactor; c) the stream 30 is guided through the gap 1300 between the first wall and the second wall of the at least twin-wall tube into the reactor; d) that stream 10 or 20 which is not guided through the inner portion of the twin-wall tube is guided past the outside of the second wall of the at least twin-wall tube such that it comes into contact with the stream 30 at the end of the at least twin-wall tube; a) the reactor has flow connection to an at least twin-wall tube comprising a first wall that encloses the inner portion of the tube and a second wall that forms a gap with the outside of the first wall; b) either the stream 10 or the stream 20 is guided through the inner portion of the at least twin-wall tube into the reactor; c) the stream 30 is guided through the gap 1300 between the first wall and the second wall of the at least twin-wall tube into the reactor; d) that stream 10 or 20 which is not guided through the inner portion of the twin-wall tube is guided past the outside of the second wall of the at least twin-wall tube such that it comes into contact with the stream 30 at the end of the at least twin-wall tube; (v) working up the gaseous crude product stream formed in the reactor to obtain the desired isocyanate, wherein the temperature of the gaseous phosgene-containing stream provided in step (ii) is adjusted to a value below the boiling point of the primary aromatic amine to be phosgenated under the conditions in step (iv).