A steam reforming process for producing synthesis gas from hydrocarbons, comprising: - a first steam reforming step, in which a gas stream of hydrocarbon feedstock, such as natural gas or methane, optionally pre-treated in a pre-treatment or pre-reforming section, is supplied to a first reforming section comprising at least a first reformer, where steam reforming reactions take place forming a partially reformed effluent; said first reformer being a gas heated reformer in which the heat necessary for the reforming reactions is provided by a fraction of the partially reformed effluent leaving said first gas heated reformer and constituting the heating fluid to provide heat to the feedstock to be reformed; - a second steam reforming step, wherein the partially reformed effluent leaving the first reforming section is supplied to a second steam reforming section comprising at least a second catalytic tube reformer, where, in an radiant chamber outside the catalytic tubes, a combustion with air, optionally pre-treated in an air pre-treatment section, is performed; wherein: - the synthesis gas leaving the second steam reforming section is sent to: a synthesis gas shift and cooling step in a synthesis gas shift and cooling section, where carbon monoxide and water present in the synthesis gas react to give hydrogen and carbon dioxide; a CO2-capturing step in a CO2-capturing section, where carbon dioxide is removed from the synthesis gas; a separating section, to obtain a high concentration hydrogen gas flow; - the synthesis gas shift and cooling section comprises a medium temperature shift unit and a low temperature shift unit connected in series and operating at respective different temperatures with intermediate cooling; - tail gases extracted downstream of the CO2-capturing section and/or from the separating section are recirculated to the second steam reforming section. - a first steam reforming step, in which a gas stream of hydrocarbon feedstock, such as natural gas or methane, optionally pre-treated in a pre-treatment or pre-reforming section, is supplied to a first reforming section comprising at least a first reformer, where steam reforming reactions take place forming a partially reformed effluent; said first reformer being a gas heated reformer in which the heat necessary for the reforming reactions is provided by a fraction of the partially reformed effluent leaving said first gas heated reformer and constituting the heating fluid to provide heat to the feedstock to be reformed; - a second steam reforming step, wherein the partially reformed effluent leaving the first reforming section is supplied to a second steam reforming section comprising at least a second catalytic tube reformer, where, in an radiant chamber outside the catalytic tubes, a combustion with air, optionally pre-treated in an air pre-treatment section, is performed; - the synthesis gas leaving the second steam reforming section is sent to: a synthesis gas shift and cooling step in a synthesis gas shift and cooling section, where carbon monoxide and water present in the synthesis gas react to give hydrogen and carbon dioxide; a CO2-capturing step in a CO2-capturing section, where carbon dioxide is removed from the synthesis gas; a separating section, to obtain a high concentration hydrogen gas flow; - the synthesis gas shift and cooling section comprises a medium temperature shift unit and a low temperature shift unit connected in series and operating at respective different temperatures with intermediate cooling; - tail gases extracted downstream of the CO2-capturing section and/or from the separating section are recirculated to the second steam reforming section.