Plastic recycling and reprocessing remain crucial to reduce plastic-related environmental pollution. Among the available approaches, solvothermal liquefaction holds particular promise for upgrading low-value plastic waste into high-value fuels and other products. However, such progress is limited by an incomplete understanding of the complex chemistry-flow coupling dynamics associated with plastic dissolution within the solvothermal reactors. This study explores two aspects related to this non-trivial coupling through numerical simulation. First, we focus on the suspension behavior and local flow conditions encountered by plastic particles in stirred tank reactors. An Euler–Lagrange CFD–DEM framework with a momentum-source rotor-disk model is employed to systematically explain how stirrer speed and solids loading affect the distributions of particle Reynolds number and strain rate. Then, the impact of polymer viscoelasticity on dissolution dynamics is investigated. Building on the finitely-extensible nonlinear elastic (FENE) theory and dumbbell model, we propose an extension of the constitutive equations for the conformation tensor volume density in spatially inhomogeneous polymer solutions, enabling the capture of viscoelastic flow features during dissolution. Canonical simulations reveal that polymer viscoelasticity can thin the boundary layer surrounding the plastic elements, amplify local concentration gradients, and increase dissolution rates by up to 35%. Moreover, high elastic stress and reduced viscosity due to the dissolved polymer not only thin the boundary layer further, but may also trigger boundary-layer instabilities, thereby enhancing dissolution efficiency.