A process for reducing coke formation during hydrocarbon upgrading reactions using a double-wall reactor, the process comprising the steps of: feeding a heated feed water to a shell-side volume of the double-wall reactor to produce a heat transfer stream, the double-wall reactor comprising: an exterior wall and an interior wall, the exterior wall and the interior wall defining the shell-side volume disposed between, a reaction section volume bounded by the interior wall, a heating element, the heating element adjacent to the exterior wall, wherein the heating element is configured to heat the heat transfer stream to create a hot water return, such that the heat transfer stream is above the critical temperature of water, wherein heat is transferred from the heat transfer stream through the interior wall to the reaction section volume, wherein the hot water return exits the shell-side volume, wherein the heat transfer stream is at a temperature greater than the critical temperature of water and is at a pressure greater than the critical pressure of water; feeding the hot water return exiting the shell-side volume of the double-wall reactor through a filter, the filter is configured to remove particulates to form a filtered water stream; mixing the filtered water stream with a heated hydrocarbon feedstock in a mixer to produce a mixed stream, wherein the heated hydrocarbon feedstock is at a pressure greater than the critical pressure of water and at a temperature greater than 50°C; feeding the mixed stream to the reaction section volume of the double-wall reactor in a flow configuration counter-current to the heat transfer stream to create a reaction flow stream; reacting the reaction flow stream at a reaction temperature in the reaction section volume to produce a reactor effluent, wherein the heat transferred to the reaction section volume from the heat transfer stream is operable to maintain the reaction temperature above the critical temperature of water; cooling the reactor effluent in a reactor cooler to produce a cooled effluent; de-pressurizing the cooled effluent in a pressure reducer to produce a depressurized effluent; separating the depressurized effluent in a phase separator to produce a gas phase product and a liquid phase product; and separating the liquid phase product in a product separator to produce a separated water stream and an upgraded hydrocarbon stream. feeding a heated feed water to a shell-side volume of the double-wall reactor to produce a heat transfer stream, the double-wall reactor comprising: an exterior wall and an interior wall, the exterior wall and the interior wall defining the shell-side volume disposed between, a reaction section volume bounded by the interior wall, a heating element, the heating element adjacent to the exterior wall, wherein the heating element is configured to heat the heat transfer stream to create a hot water return, such that the heat transfer stream is above the critical temperature of water, wherein heat is transferred from the heat transfer stream through the interior wall to the reaction section volume, wherein the hot water return exits the shell-side volume, wherein the heat transfer stream is at a temperature greater than the critical temperature of water and is at a pressure greater than the critical pressure of water; an exterior wall and an interior wall, the exterior wall and the interior wall defining the shell-side volume disposed between, a reaction section volume bounded by the interior wall, a heating element, the heating element adjacent to the exterior wall, wherein the heating element is configured to heat the heat transfer stream to create a hot water return, such that the heat transfer stream is above the critical temperature of water, wherein heat is transferred from the heat transfer stream through the interior wall to the reaction section volume, wherein the hot water return exits the shell-side volume, wherein the heat transfer stream is at a temperature greater than the critical temperature of water and is at a pressure greater than the critical pressure of water; feeding the hot water return exiting the shell-side volume of the double-wall reactor through a filter, the filter is configured to remove particulates to form a filtered water stream; mixing the filtered water stream with a heated hydrocarbon feedstock in a mixer to produce a mixed stream, wherein the heated hydrocarbon feedstock is at a pressure greater than the critical pressure of water and at a temperature greater than 50°C; feeding the mixed stream to the reaction section volume of the double-wall reactor in a flow configuration counter-current to the heat transfer stream to create a reaction flow stream; reacting the reaction flow stream at a reaction temperature in the reaction section volume to produce a reactor effluent, wherein the heat transferred to the reaction section volume from the heat transfer stream is operable to maintain the reaction temperature above the critical temperature of water; cooling the reactor effluent in a reactor cooler to produce a cooled effluent; de-pressurizing the cooled effluent in a pressure reducer to produce a depressurized effluent; separating the depressurized effluent in a phase separator to produce a gas phase product and a liquid phase product; and separating the liquid phase product in a product separator to produce a separated water stream and an upgraded hydrocarbon stream.