sent = 'A survey of user opinion of computer system response time'
tagged = pos_tag(word_tokenize(sent))
chunks = []
current_chunk = []
for word, pos in tagged:
    if pos.startswith('N'):
        current_chunk.append((word, pos))
else:
    if current_chunk:
        chunks.append(current_chunk)
current_chunk = []
chunks
for i in chunks:
    pass
How to parse sentences based on lexical content (phrases) with Python-NLTK