A method for producing welded connections between inner tubes and tube support plates (12, 14) of a tube bundle for a product-to-product shell-and-tube heat exchanger by means of an auxiliary device, wherein the relevant tube support plate (12, 14) is welded by its end face that faces the inner tubes, said method being carried out with the following steps (i) to (iii): (i) orienting the inner tubes with assigned plate inner bores arranged in the tube support plate (12, 14) in such a way that the inner tubes and the plate inner bores are aligned with one another; (ii) fixing the relevant inner tube and the tube support plate (12, 14) to one another in an unwelded starting position by means of a form fit that is effective radially and in opposing directions, wherein more than one plate inner bore is defined and therefore a tube layout pattern is established by means of the tube support plate (12, 14) for assigned inner tubes arranged in parallel with one another; (iii) immovably fixing a number (n) of inner tubes required in each case to one another as a whole according to the tube layout pattern and in the relevant form fit with the tube support plate (12, 14) by means of the auxiliary device, characterized by the following additional steps (iv) to (viii): (iv) pressing the end face of the tube support plate (12, 14) against the end face of the inner tubes in the direction of the inner tube longitudinal axes of the inner tubes for the duration of the welding operations for all the inner tubes with a first form fit, which is effective both radially and axially and in each case in opposing directions, wherein the plate inner bore and a tube inner bore of the inner tube have inner diameters of equal size, (v) immovably fixing the number (n) of inner tubes required in each case to one another as a whole according to the tube layout pattern by means of a second, detachable form fit of the auxiliary device, (vi) making a circumferential round weld orbitally, starting from the plate and the tube inner bore (17, 16a), in a single pass and continuously from radially inside to radially outside, wherein at least one axial form fit depth (B 1) of the first form fit (F 1) is covered by a weld width (b) of the circumferential round weld, (vii) welding the inner tubes to the tube support plate (12, 14) one after the other, and (viii) detaching and removing the auxiliary device from the welded tube arrangement. (i) orienting the inner tubes with assigned plate inner bores arranged in the tube support plate (12, 14) in such a way that the inner tubes and the plate inner bores are aligned with one another; (ii) fixing the relevant inner tube and the tube support plate (12, 14) to one another in an unwelded starting position by means of a form fit that is effective radially and in opposing directions, wherein more than one plate inner bore is defined and therefore a tube layout pattern is established by means of the tube support plate (12, 14) for assigned inner tubes arranged in parallel with one another; (iii) immovably fixing a number (n) of inner tubes required in each case to one another as a whole according to the tube layout pattern and in the relevant form fit with the tube support plate (12, 14) by means of the auxiliary device, (iv) pressing the end face of the tube support plate (12, 14) against the end face of the inner tubes in the direction of the inner tube longitudinal axes of the inner tubes for the duration of the welding operations for all the inner tubes with a first form fit, which is effective both radially and axially and in each case in opposing directions, wherein the plate inner bore and a tube inner bore of the inner tube have inner diameters of equal size, (v) immovably fixing the number (n) of inner tubes required in each case to one another as a whole according to the tube layout pattern by means of a second, detachable form fit of the auxiliary device, (vi) making a circumferential round weld orbitally, starting from the plate and the tube inner bore (17, 16a), in a single pass and continuously from radially inside to radially outside, wherein at least one axial form fit depth (B 1) of the first form fit (F 1) is covered by a weld width (b) of the circumferential round weld, (vii) welding the inner tubes to the tube support plate (12, 14) one after the other, and (viii) detaching and removing the auxiliary device from the welded tube arrangement.