A method for washing an oligomerization reactor using by-product solvent recovered from the reactor can include: catalytically converting a monomer in a reactor section in a reaction mode in the presence of a catalyst to form a product stream comprising an oligomer, a by-product solvent, and a polymeric by-product; separating the product stream into a first fraction comprising the oligomer and a second fraction comprising a mixture of the by-product solvent and the polymeric by-product; and separating, in a thin film evaporator, the second fraction into a third fraction comprising the by-product solvent and a fourth fraction comprising the polymeric by-product.