Processes for production of olefins from hydrocarbon feedstocks are provided. In one aspect, the processes of the present invention utilize coils passing through a pyrolysis furnace to partially convert a hydrocarbon feedstock to olefins, followed by further conversion of the hydrocarbon feedstock in an adiabatic reactor. A portion of the coils in the pyrolysis furnace carry the hydrocarbon feedstock and the remainder carry steam only. After a selected period of time, the material flowing through the coils is switched. By flowing steam through the coils that had previously contained the hydrocarbon feedstock, on-line decoking can occur. In another aspect, a high temperature reactor is used to convert methane or natural gas to olefins.