You can Replace() the comma.
cols6[j].InnerText = cols6[j].InnerText.Replace(",", "");
For the WriteLine() you could also go like this:
sw.WriteLine(cols[j].InnerText + "," + cols2[j].InnerText + "," + cols3[j].InnerText + "," +
cols4[j].InnerText + "," + cols5[j].InnerText + "," + cols6[j].InnerText.Replace(",", "") + "," + cols7[j].InnerText + ",822");