Complex sentence: Schumann gained quick success as a symphonic composer following his orchestral debut with his warmly-received First Symphony, which was composed in 1841 and premiered in Leipzig with Felix Mendelssohn conducting.
Split into simpler sentences: Schumann gained quick success as a symphonic composer following his orchestral debut with his warmly-received First Symphony. In 1841 Schumann completed his First Symphony, and had it premiered in Leipzig with Felix Mendelssohn conducting.