Systems and methods are provided for integration of a reactor for polyolefin pyrolysis with the effluent processing train for a steam cracker. The polyolefins can correspond to, for example, polyolefins in plastic waste. Integrating a process for polyolefin pyrolysis with a steam cracker processing train can allow a mixture of polymers to be converted to monomer units while reducing or minimizing costs and/or equipment footprint. This can allow for direct conversion of polyolefins to the light olefin monomers in high yield while significantly lowering capital and energy usage due to integration with a steam cracking process train. The integration can be enabled in part by selecting feeds with appropriate mixtures of various polymer types and/or by limiting the volume of the plastic waste pyrolysis product relative to the volume from the steam cracker(s) in the steam cracking process train. By selecting plastic waste and/or other polyolefin sources with an appropriate mixture of polyolefins as the feedstock, the resulting polyolefin pyrolysis product can be separated in a steam cracking process train to produce separate fractions for various polymer grade small olefin products.