Systems and methods for thermally decomposing hydrocarbon feedstock may comprise a hydrocarbon feedstock, a non-oxidizing carrier gas, one or more heat exchangers, and a reaction chamber. The carrier gas may be used to transfer heat to the hydrocarbon feedstock. The heat exchanger(s) may be configured to heat the carrier gas. And the reaction chamber may be configured to receive hydrocarbon feedstock and heated carrier gas. Inside the reaction chamber, the hydrocarbon feedstock and the heated carrier gas may mix with one another causing the thermal decomposition reaction. The thermal decomposition reaction occurs in a substantially oxidant-free environment thereby eliminating or greatly reducing the production of carbon oxide byproducts. Hydrogen gas may be separated from a gaseous product stream that is thereafter collected from the reaction chamber. A portion of the gaseous product stream may be thermally coupled to the carrier gas and may thereafter be recycled through the system.