Method of producing oligomer

A method of producing an oligomer, the method comprising:

• supplying a monomer stream including a monomer and a solvent stream to a reactor to perform an oligomerization reaction using the monomer to produce a reaction product including an oligomer and a by-product, wherein the by-product comprises a polymer;
• supplying a discharge stream of the reactor to a separation device, and supplying an upper discharge stream of the separation device including an unreacted monomer to the reactor and supplying a lower discharge stream of the separation device to a settling tank;
• settling the polymer in the settling tank and removing the polymer, and supplying the lower discharge stream of the separation device from which the polymer is removed to a high-boiling point separation column;
• removing a high-boiling point material from a lower discharge stream of the high-boiling point separation column and supplying an upper discharge stream of the high-boiling point separation column including the oligomer to a solvent separation column;
• and
• separating a solvent and the oligomer in the solvent separation column,
• wherein layer separation into an upper layer liquid including the solvent and a lower layer liquid including the oligomer is performed in the settling tank, and an upper layer liquid stream is directly supplied to the solvent separation column and a lower layer liquid stream is supplied to the high-boiling point separation column.