Process for producing a liquid nitrogen stream, a gaseous nitrogen stream, a gaseous stream which is rich in helium and a denitrided stream of hydrocarbons from a feed stream which contains hydrocarbons, nitrogen and helium, the process comprising the following steps: - expanding the feed stream in order to form an expanded feed stream; - dividing the expanded feed stream into a first introduction stream and a second introduction stream; - cooling the first introduction stream within an upstream heat exchanger by heat exchange with a gaseous refrigerant stream which is obtained by dynamic expansion in a cooling cycle in order to obtain a first cooled introduction stream; - cooling the second introduction stream by means of a first downstream heat exchanger in order to form a second cooled introduction stream; - introducing the first cooled introduction stream and the second cooled introduction stream into a fractionating column which comprises a plurality of theoretical fractionating stages; - tapping at least one reboiling stream and circulating the reboiling stream in the first downstream heat exchanger in order to cool the second introduction stream; - tapping, at the bottom of the fractionating column, a bottom stream which is intended to form the denitrided stream of hydrocarbons; - tapping, at the head of the fractionating column, a head stream which is rich in nitrogen; - reheating the head stream which is rich in nitrogen by means of at least one second downstream heat exchanger (54, 56) in order to form a reheated stream which is rich in nitrogen; - tapping and expanding a first portion of the reheated stream which is rich in nitrogen in order to form the gaseous nitrogen stream; - compressing a second portion of the reheated stream which is rich in nitrogen in order to form a compressed, recycled nitrogen stream, and cooling the compressed, recycled nitrogen stream by means of circulation through the first downstream heat exchanger and the or each second downstream heat exchanger (54, 56); - liquefying and partially expanding the recycled nitrogen stream in order to form an expanded nitrogen rich stream; - introducing at least a portion (106; 146) obtained from the expanded nitrogen rich stream into a first separation container; - recovering the gaseous head stream from the first separation container in order to form the helium rich stream; - recovering the liquid stream from the bottom of the first separation container and separating that liquid stream into a liquid nitrogen stream and a first reflux stream; - introducing the first reflux stream as reflux into the head of the fractionating column. - expanding the feed stream in order to form an expanded feed stream; - dividing the expanded feed stream into a first introduction stream and a second introduction stream; - cooling the first introduction stream within an upstream heat exchanger by heat exchange with a gaseous refrigerant stream which is obtained by dynamic expansion in a cooling cycle in order to obtain a first cooled introduction stream; - cooling the second introduction stream by means of a first downstream heat exchanger in order to form a second cooled introduction stream; - introducing the first cooled introduction stream and the second cooled introduction stream into a fractionating column which comprises a plurality of theoretical fractionating stages; - tapping at least one reboiling stream and circulating the reboiling stream in the first downstream heat exchanger in order to cool the second introduction stream; - tapping, at the bottom of the fractionating column, a bottom stream which is intended to form the denitrided stream of hydrocarbons; - tapping, at the head of the fractionating column, a head stream which is rich in nitrogen; - reheating the head stream which is rich in nitrogen by means of at least one second downstream heat exchanger (54, 56) in order to form a reheated stream which is rich in nitrogen; - tapping and expanding a first portion of the reheated stream which is rich in nitrogen in order to form the gaseous nitrogen stream; - compressing a second portion of the reheated stream which is rich in nitrogen in order to form a compressed, recycled nitrogen stream, and cooling the compressed, recycled nitrogen stream by means of circulation through the first downstream heat exchanger and the or each second downstream heat exchanger (54, 56); - liquefying and partially expanding the recycled nitrogen stream in order to form an expanded nitrogen rich stream; - introducing at least a portion (106; 146) obtained from the expanded nitrogen rich stream into a first separation container; - recovering the gaseous head stream from the first separation container in order to form the helium rich stream; - recovering the liquid stream from the bottom of the first separation container and separating that liquid stream into a liquid nitrogen stream and a first reflux stream; - introducing the first reflux stream as reflux into the head of the fractionating column.