Processes and apparatuses for producing polymer particles with a solid state polycondensation reactor and an underwater pelletization unit. The apparatuses use a pre-conditioning zone to adjust a temperature, crystallization in addition to dust, acetaldehyde and water content of the particles from a crystallization bin. Various inert gas streams can be provided from a purification unit to remove dust, acetaldehyde, water and adjust temperature and crystallinity of the particles, as also move the particles. The precondition zones have stages that allow for the particles to accurately achieve the desired SSP reactor inlet conditions.