System for measuring a specific temperature for at least one reforming tube present in a steam methane reforming furnace in which: • reforming tubes disposed vertically in rows are supplied at the inlet with a gaseous reaction mixture and produce a synthesis gas at the tube outlet, • a set of longitudinal tubular manifolds is disposed in a plane perpendicular to the rows of tubes, beneath said rows, so as to collect the synthesis gas produced, the tubes of a given row opening into a longitudinal manifold that is associated therewith, all the longitudinal manifolds opening into a transverse manifold, all the longitudinal manifolds and the transverse manifold forming the system for collecting the synthesis gas, characterized in that the system comprises, for at least one row of tubes, at least • a means for measuring at least one synthesis gas temperature at the tube outlet, the means being disposed along the length of the manifold associated with said row of tubes and comprising at least: o a first part of tubular shape disposed along the length of said longitudinal manifold and provided with at least one temperature sensor so as to acquire temperature data, which sensor is positioned in the synthesis gas at the mouth of a reforming tube of the row of which the temperature of the outlet synthesis gas is measured; o a second part extending the first part that is able to ensure the transfer of the temperature data acquired by the sensor to the outside of the longitudinal manifold, and as far as the third part; o a third part comprising instrumentation for calculating the temperature from the data acquired by the first part of the means and transferred by the second part of the means; • a means for positioning the temperature sensor in the longitudinal manifold that ensures its positioning in the flow of synthesis gas at the mouth of the tube, before it is mixed with the synthesis gas coming from the other tubes opening into the manifold; • a sealed outlet means for the second part of the measurement means towards the outside of the system for collecting the synthesis gas. • reforming tubes disposed vertically in rows are supplied at the inlet with a gaseous reaction mixture and produce a synthesis gas at the tube outlet, • a set of longitudinal tubular manifolds is disposed in a plane perpendicular to the rows of tubes, beneath said rows, so as to collect the synthesis gas produced, the tubes of a given row opening into a longitudinal manifold that is associated therewith, all the longitudinal manifolds opening into a transverse manifold, all the longitudinal manifolds and the transverse manifold forming the system for collecting the synthesis gas, • a means for measuring at least one synthesis gas temperature at the tube outlet, the means being disposed along the length of the manifold associated with said row of tubes and comprising at least: o a first part of tubular shape disposed along the length of said longitudinal manifold and provided with at least one temperature sensor so as to acquire temperature data, which sensor is positioned in the synthesis gas at the mouth of a reforming tube of the row of which the temperature of the outlet synthesis gas is measured; o a second part extending the first part that is able to ensure the transfer of the temperature data acquired by the sensor to the outside of the longitudinal manifold, and as far as the third part; o a third part comprising instrumentation for calculating the temperature from the data acquired by the first part of the means and transferred by the second part of the means; o a first part of tubular shape disposed along the length of said longitudinal manifold and provided with at least one temperature sensor so as to acquire temperature data, which sensor is positioned in the synthesis gas at the mouth of a reforming tube of the row of which the temperature of the outlet synthesis gas is measured; o a second part extending the first part that is able to ensure the transfer of the temperature data acquired by the sensor to the outside of the longitudinal manifold, and as far as the third part; o a third part comprising instrumentation for calculating the temperature from the data acquired by the first part of the means and transferred by the second part of the means; • a means for positioning the temperature sensor in the longitudinal manifold that ensures its positioning in the flow of synthesis gas at the mouth of the tube, before it is mixed with the synthesis gas coming from the other tubes opening into the manifold; • a sealed outlet means for the second part of the measurement means towards the outside of the system for collecting the synthesis gas.