NOVELTY - A cascade-type liquefied natural gas apparatus having at least three sequential cooling cycles is provided. The methane refrigerant is compressed, the compressed methane refrigerant is separated into a primary compressed methane refrigerant portion and a secondary compressed methane refrigerant portion, the heavies-reduced natural gas stream is introduced into an open-loop methane refrigeration cycle, and a portion of the open-loop methane refrigerant is combined with a portion of the primary natural gas stream, to form a liquefied natural gas product.    USE - Method for liquefying natural gas stream (claimed).    ADVANTAGE - The method enables economical and efficient liquefaction of natural gas stream by a simple process.    DETAILED DESCRIPTION - A cascade-type liquefied natural gas apparatus having at least three sequential cooling cycles, each employing a different refrigerant is provided. At least three sequential cooling cycles comprise a primary cooling cycle with a primary refrigerant, a secondary cooling cycle with a secondary refrigerant, and a tertiary cooling cycle with a third refrigerant. The tertiary refrigerant is a methane refrigerant. The tertiary cooling cycle has an open-loop methane refrigeration cycle and comprises methane. The natural gas stream in primary cooling cycle is cooled with primary refrigerant, the natural gas stream in the secondary cooling cycle is cooled with secondary refrigerant, and the natural gas stream is introduced to heavies removal column for separating the natural gas stream into a heavies stream and a heavies-reduced natural gas stream. The methane refrigerant is compressed to form a compressed methane refrigerant, the compressed methane refrigerant is separated into a primary compressed methane refrigerant portion and a secondary compressed methane refrigerant portion, and the heavies-reduced natural gas stream is introduced into an open-loop methane refrigeration cycle by combining the heavies-reduced natural gas stream with the primary compressed methane refrigerant portion to form a combined stream. The combined stream is introduced into a methane economizer to form a primary natural gas stream. The secondary compressed methane refrigerant portion is introduced into a chiller to form a cooled methane refrigerant product. The cooled methane refrigerant product is introduced to a methane refrigerant accumulation vessel in which liquid level is formed in the methane refrigerant accumulation vessel, and liquid level in the methane refrigerant accumulation vessel is controlled by adjusting flow of the primary compressed methane refrigerant portion with respect to a flow of the secondary compressed methane refrigerant portion to maintain the liquid level in the methane refrigerant accumulation vessel. The cooled methane refrigerant product from the methane refrigerant accumulation vessel is introduced into the methane economizer to form an open-loop methane refrigerant by indirect heat exchange with the open-loop methane refrigerant, the primary natural gas stream is cooled in several separate heat exchangers through indirect heat exchange with the open-loop methane refrigerant, and a portion of the open-loop methane refrigerant is combined with a portion of the primary natural gas stream, to form a liquefied natural gas product.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of cascade-type liquefied natural gas apparatus.    Chiller (2)    Expansion valve (12)    Compressor (18)    Cooler (20)    Conduit (302)