Methods and systems of the present invention use cellulose-containing materials, which may include post-consumer waste garments, scrap fabric and/or various biomass materials as a raw feed material to produce isolated cellulose molecules having desirable properties that can be used in the textile and apparel industries, and in other industries. A multi-stage process is provided, in which cellulose-containing feed material is subjected to one or more pretreatment stages, followed by a pulping treatment, to isolate cellulose molecules. The isolated cellulose molecules may be used in a variety of downstream applications. In one application, isolated cellulose molecules are extruded to provide regenerated cellulose fibers having desirable (and selectable) properties that are usable in various industrial applications, including textile production.