Catalysis of a multistep reaction that produce non-ribosomal peptides. The key chain-building reaction, a C-N bond-forming reaction, involves the generation of the characteristic peptide bond by nucleophilic attack of the amino group of an amino-acyl donor unit covalently bound to a downstream peptidyl carrier protein module (amino acyl-S-PCP) on the acyl group of an upstream electrophilic acyl- or peptidyl acyl-S-PCP chain, catalyzed by a condensation (C) domain. Supplementing these core chain-elongation domains are variable numbers of auxiliary domains that are responsible for modification of the growing polypeptide chain by a small set of iterated reactions including epimerization, N-methylation, and heterocyclization. [ http://www.ncbi.nlm.nih.gov/pubmed/10631508 http://www.ncbi.nlm.nih.gov/pubmed/17502372 ]