Processes and systems for hydrocarbon pyrolysis. In some embodiments, a hydrocarbon can be heated within a convection section of a steam cracking furnace and combined with an aqueous fluid to produce a heated mixture. A heavy feed that includes a plastic material can be introduced into a vessel and a portion of the plastic material can be cracked therein. Liquid and vapor effluents exiting the vessel can be obtained. At least a portion of the liquid effluent can be heated to produce a heated fluid stream that can be recycled to the vessel. The vapor effluent can be combined with the heated mixture to produce a combined mixture that can be heated within the convection section to produce a heated combined mixture. At least a portion of the heated combined mixture can be cracked within a radiant section of the steam cracking furnace to produce a steam cracker effluent.