CentOS 7 NodeJs Application - 502 Bad Gateway

Question

Cannot reach my web service which is running on Nginx/CentOS 7. 502 Bad Gateway error occurs when I try to request. Developed with NodeJS. For more information I shared my configuration files.

Also codes work on local machine but not working on the server.

Server NodeJS version v10.10.0 Local NodeJS version v9.3.0

nginx config

upstream node_server {
server 127.0.0.1:5000 fail_timeout=0;
server 127.0.0.1:5001 fail_timeout=0;
}

server {
    listen 80 default_server;
    listen [::]:80 default_server;

    index index.html index.htm;

    server_name alpha;

    location / {
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_redirect off;
        proxy_buffering off;
        proxy_pass http://node_server;
    }

    location /public/ {
        root /opt/app;
    }
}

Service file located at /etc/systemd/system/node-app-1.service

[Service]
ExecStart=/usr/bin/node /opt/app/app.js
Restart=always
StandardOutput=syslog
StandardError=syslog
SyslogIdentifier=node-app-1
User=root
Group=root
Environment=NODE_ENV=production PORT=5000

[Install]
WantedBy=multi-user.target

Error detail located in error.log file

2018/09/16 03:30:59 [error] 3102#0: *16 connect() failed (111: Connection refused) while connecting to upstream, client: <MYIPADDRESS>, server: alpha, request: "GET / HTTP/1.1", upstream: "<SERVERIPADDRESS>", host: "<SERVERIPADDRESS>"

I tried to run npm run start command inside my root folder and it worked fine.

http & https firewall enabled

Origin Server is up

Approximately I am searching this issue for 5-6 hours and could not find any solution to take a deep breath.

I could not find same question on the platform. If exist, please let me know and mark this question as duplicate.