I am trying to scrap a website where targeted items are populated using document.write method. How can I get full browser html rendered version of the website in the Scrapy?
You can't do this, as scrapy will not execute the JavaScript code.
What you can do:

Rely on a headless browser like Selenium, which will execute the JavaScript. Afterwards, use XPath (or simple DOM access) like before to query the web page after executing the page.
Understand where the contents come from, and load and parse the source directly instead. Chrome Dev Tools / Firebug might help you with that, have a look at the "Network" panel that shows fetched data.
Especially look for JSON, sometimes also XML.
Scrapy: scraping website where targeted items are populated using document.write